An orthogonal frequency division multiplex (OFDM) system and a MIMO (Multiple Input Multiple Output) are core techniques in future radio communication. Merging an OFDM system with a MIMO system makes it possible to effectively improve frequency spectrum efficiency and simplify the system. In the MIMO system, data is transmitted from a plurality of radio physical channels simultaneously, and so, a preamble sequence is generated in the frequency domain, space domain and time domain simultaneously, and therefore the MIMO system is more complicated than a SISO (Single Input Single Output) system.
In a radio communication system, pilots and training sequences have quite important roles. This radio communication system generates a variety of pilots for channel estimation, timing synchronization and frequency offset estimation (Non-Patent Document 1) and the like, applies different methods for generating their respective preamble sequences and uses channel resources different from each other. To guarantee the accuracy of synchronization, frequency offset and channel estimation, pilots use more resources if preamble sequences and data sequences have the same length. For example, pilot energy is higher than data energy, and so, in the OFDM communication system, a carrier interval of short training sequences is four times the subcarrier interval of normal symbols.
The most part of studies on the MIMO system focus attention on the generation of a preamble sequence when channel estimation is carried out. Those studies are targeted at a MIMO channel environment and determine a scheme of generating a preamble sequence for channel estimation, thereby obtaining an optimum solution for the preamble sequence. On the other hand, to eliminate a frequency offset, a method of eliminating a frequency offset of a SISO system is generally used (Non-Patent Document 2). FIG. 1 shows a pilot configuration in an IEEE802.11 protocol. The frequency offset elimination pilot and the channel estimation pilot use different symbol resources in the IEEE802.11 protocol, so that a long preamble sequence of 8 ms is provided for the channel estimation pilot, and ten short preamble sequences include four training sequences for frequency offset estimation.    Non-Patent Document 1: Richard van Nee, Ramjee Prased “OFDM wireless multimedia communications, Boston, London: Artech House, 2000”    Non-Patent Document 2: IEEE C802.16e-04/192, “Preamble design to improve MIMO support”, 2004.6.26